Synthesis of well-defined cyclic poly(N-isopropylacrylamide) via click chemistry and its unique thermal phase transition behavior

We report on the preparation of well-defined cyclic poly (N-isopropylaerylamide) (cyclic-PNIPAM) via click chemistry and its unique thermal phase transition behavior as compared to the linear counterpart. alpha-Alkyne-omega-azido heterodifunctional PNIPAM precursor. (linear-PNIPAM-N-3) was prepared by atom transfer radical polymerization (ATRP) of N-isopropylacrylamide in 2-propanol using propargyl 2-chloropropionate as the initiator; followed by reacting with NaN3 to transform the terminal chloride into azide group. The subsequent end-to-end intramolecular coupling reaction under high dilution and "click" conditions leads to efficient preparation of narrow-disperse cyclic-PNIPAM. Gel permeation chromatography (GPC), H-1 NMR, Fourier transform infrared (FT-IR), and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry all confirmed the complete transformation of linear-PNIPAM-N-3 to cyclic-PNIPAM. The thermal phase transition behavior of cyclic-PNIPAM was investigated by temperature-dependent turbidity measurements and micro-differential scanning calorimetry (micro-DSC) and compared to that of linear-PNIPAM-N-3 with the same molecular weight. The former possesses lower critical solution temperatures (LCSTs), more prominent concentration dependences of LCST values and cloud points (CPs), broader thermal phase transition range, and prominently lower enthalpy changes (Delta H). The above differences in thermal phase transition behaviors between cyclic- and linear-PNIPAM should be due to the absence of chain ends and stringent restrictions on backbone conformations in the former.